1. Field of the Invention
The present invention relates to a solid state image pick-up device and, more particularly, it relates to the structure of a solid state image pick-up device picking up an image of an object that is directly put on the surface of the solid state image pick-up device.
2. Description of the Related Art
Conventionally, when an image of a fingerprint is outputted by a fingerprint image input system that uses solid state image pick-up devices such as a CCD image sensor or a CMOS image sensor, the system has been required to have optical parts such as a lens, a prism or a fiber in addition to the above solid state image pick-up device. For that reason, the system had to have substantial space to accommodate the rather expensive optical part, including its fixing position. Thus, the system has not been easily thinned, miniaturized or cost-reduced.
Furthermore, the image obtained through the optical part processing was apt to include image distortion.
To solve the above-stated problems, there is provided a conventional fingerprint image photographing method in U.S. patent application Ser. No. 09/494,506 that the finger (fingerprint) to be photographed is placed near or put directly on the surface of the solid state image pick-up device. This method need not employ optical parts such as a lens or a prism and enables the fingerprint image input system to be thinned, miniaturized and cost-reduced. In addition, this method also eliminates a distortion caused by an optical processing.
On the other hand, a conventional method is known that uses an electrostatic capacitive sensor for performing an image input of a fingerprint. According to this method, a finger is directly put on the surface (upper surface) of a chip, on which a capacitor electrode is formed, to get the fingerprint image. The electrostatic capacitive sensor does not need to incorporate the above-mentioned optical parts but is fundamentally weak against the stress caused by electrostatic discharge. In addition, it does not show a good sensitivity in getting the fingerprint image since it is affected easily by the degree of finger dryness.
FIG. 1 shows an example in which a CMOS image sensor is used to photograph a fingerprint image by putting the finger directly on the surface (upper surface) of a chip 110. As shown in FIG. 1, a CMOS sensor is formed so as to include photo detective regions 102 arrayed in a matrix in a silicon substrate 101. In this structure, signal electric charges collected in the photo detective region 102 are transferred to a peripheral MOSFET 108 through a wiring 112. The surface of the silicon substrate 101 including the peripheral MOSFET 108 and the wiring 112 is covered by an interlayer insulating film 109 consisting of a silicon dioxide.
When a finger 120 is put directly on the surface of the interlayer insulating film 109 on the silicon substrate 101, lights 130 and 131 irradiated from a fluorescent lamp or an LED are incident on crista lines of a fingerprint 121 and reflected thereon so as to enter the inner portion of the silicon substrate 101. After that, the signal electric charges are generated in the inner portion of the silicon substrate 101 and collected by the photo detective region 102, and finally outputted as a fingerprint image.
However, the problem with this method is that the chip 110 is easily, physically broken. This problem occurs as follows: first, the finger 120 is put directly on the surface of the chip 110 (the problem becomes serious especially when the finger is put on the photo detective region 102), secondly the surface of the chip 110 is scrubbed or damaged by a finger nail, resulting in a chip surface scratch on the chip or at worst the breakage of the chip together with the elements included therein.
Furthermore, another problem with this method is that the chip is apt to deteriorate its electric characteristics by the diffusion of contamination materials to the inner portion of the chip. This phenomenon is caused by the adherence of various contaminant materials to the revealed surface of the chip 110. Especially when sodium or the like adheres to the surface of the chip, it rapidly diffuses into the chip 110 and deteriorates the characteristics of the elements. Moreover, one more problem is seen with this method. When the finger 120 is put directly on the surface of the chip 110, it is located very near the wiring 112. The static electricity generated when the interlayer insulating film 109 is touched by the finger 120 discharges through the wiring 112 destroying the gate insulation film of the peripheral MOSFET 108 of FIG. 1. The destruction is caused as follows: first, as shown in FIG. 1, the finger 120 is put directly on the interlayer insulating film above the MOSFET 108 or the wiring 112; secondly, excess Electro Static Discharge (ESD) included in the finger 120 is applied to the surface of the chip 110; and finally, these elements, like MOSFET 108, are destroyed by ESD through the wiring 112 since the elements are usually not protected electrically by a protection circuit against ESD.